Applications that utilize swept-wavelength lasers rely on precise measurements and control of the laser. A variety of artifacts may hinder the measurement process. For example, etalons formed from small air gaps in the optical path can lead to intensity modulation from interferences, especially when the laser is tuned over a range of wavelengths. The resulting intensity modulation creates an uncertainty in the measurement of insertion loss or polarization dependent loss. Likewise, stray reflections from other elements in the optical system, for example lenses or beam-splitters, can introduce additional reflections that interfere with the reference signal. The additional interference signals add noise to the signal from the sample. In interferometric imaging such as Optical Coherence Tomography, secondary reflections from tissue create image ghosting when the coherence length is sufficiently in excess of the primary imaging depth.
In the field of telecommunications, lasers for testing and measuring may offer a version of coherence control. Lasers for telecommunications testing are often external cavity diode lasers, based on a semiconductor gain section. Coherence control in External Cavity Tunable Laser Sources (EC-TLS) often consists of modulating the gain current, which causes temperature variations in the gain medium. Temperature variation in the gain medium modulates the cavity length, which in turn changes the laser wavelength. The modulation bandwidth is limited by the thermal time constants of the heat propagating through the semiconductor. Further, modulating the current through the gain medium also modulates the output power of the laser, which may introduce noise to the measurement application.
Based on the foregoing, an alternative method of coherence control is desirable.